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Al2O3 catalysts
Solid State Communications 123 (2002) 161–166 www.elsevier.com/locate/ssc
In situ X-ray diffraction study of carbon nanotubes and filaments during their formation over Co/Al2O3 catalysts J.P. Pinheiroa,*, M.C. Schoulera, E. Dooryheeb a
Laboratoire de Thermodynamique et Physico-Chimie Me´tallurgiques, UMR 5614 CNRS-INPG-UJF, Grenoble, France Laboratoire de Cristallographie, CNRS UPR5031, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France
b
Received 25 March 2002; accepted 28 April 2002 by P. Burlet
Abstract Depending on the experimental conditions, the CO disproportionation reaction over well-calibrated Co/Al2O3 catalysts leads to the formation of carbon with two distinct morphologies: carbon nanotubes (with cylindrical carbon layers parallel to the tube axis) and carbon filaments (constituted of carbon layers with a truncated cone shape). The structural differences between these two morphologies were investigated by X-ray diffraction using synchrotron X-ray radiation. In situ X-ray diffraction analyses performed during the carbon growth at 600 8C show that the (002) inter-layer spacing is larger by a few percents in carbon nanotubes than in carbon filaments. d(002) is, respectively, equal to 0.3486 and 0.3469 nm for the nanotubes and filaments. The variation of d(002) with the graphene layer stacking geometry was confirmed by additional measurements performed at room temperature on samples prepared ex situ. The evolution of the cobalt catalyst during the carbon growth was also investigated. The results of this study indicate that the catalyst deactivation is not the result of a bulk transformation of the cobalt particles. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 81.07.De; 61.12.Ld Keywords: A. Carbon nanotab; A. Carbon filament; B. Catalytic growth; C. X-ray scattering; E. Synchrotron radiation
1. Introduction Due to their remarkable properties, carbon nanotubes show promise for numerous potential applications including gas absorption, field emission in flat-panel displays or use as nanoscale electronic devices [1– 3]. Among all the methods currently used for nanotube preparation, the chemical vapor deposition techniques (CVD, HFCVD, etc.) are probably the most promising for large-scale, low-cost production of such structures. A major feature of these techniques is that they allow the controlled production of a wide variety of shapes and morphologies [4]. In the present work, we will mainly focus on two kinds of carbon morphologies: nanotubes and filaments. We shall call nanotubes carbon structures in which the carbon layers consist in coaxial cylinders and carbon filaments structures in which the carbon layers have a truncated cone shape (Fig. 1). * Corresponding author. E-mail address: [email protected] (J.P. Pinheiro).
In a previous study [5], we demonstrated that it is possible to select the nature of the carbon structure by applying appropriate gas mixtures on well-calibrated Co/ Al2O3 catalyst between 500 and 600 8C. This work showed indeed that the carbon deposit issued from disproportionation of pure CO was exclusively composed of nanotubes. Conversely, when H2 was added to CO, only filaments were produced. It was observed, in that case, that the inclination between the carbon layers and the tube axis depends on the CO/H2 ratio and can be increased by increasing the amount of hydrogen in the mixture. It is to be noted that mixtures of both morphologies were never observed. Since their discovery by Iijima in the early 1990s [6], the carbon nanotubes have been the subject of many structural studies. X-ray diffraction studies performed by Saito et al. [7] revealed that the distance between the carbon layers in these structures was larger by a few percents than that in bulk graphite. Saito et al. estimated that the average intershell spacing is equal to 0.344 nm. High resolution transmission electron microscopy (HRTEM) and electron
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 1 8 9 - 8
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Fig. 1. Structure of the carbon nanotubes and carbon filaments.
diffraction studies conducted by Bretz et al. [8] and Sun et al. [9] gave a larger estimation of this distance (0.375 and 0.360 nm, respectively). This disparity in values was interpreted as the result of a possible dependence on the tube size by Kiang et al. [10], who proposed a model taking into account the nanotube diameter. It is to be noted that the existing diffraction data essentially deal with carbon nanotubes and that only a limited amount of information is available on filaments. The first objective of the present study is therefore to measure how the d(002) lattice spacing varies with the graphene layer stacking geometry. A progressive deactivation of the catalyst is generally observed during the carbon growth when CO is used as the carbon-bearing gas. In the case of iron-containing catalysts, it has been demonstrated that carburization (formation of Fe3C) is strongly involved in the deactivation process [11, 12]. Some authors also attribute the deactivation to an additional phenomenon: the carbon coating over the surface of the catalytic particle [13,14]. This surface carbon layer would hinder the contact between the gas phase and the particle and would therefore have a negative influence on the catalyst activity. The second objective of the present study is to search for the possible transformation of the cobalt particles into any cobalt carbide phase in the case of a cobalt-containing catalyst. Since the cobalt carbides are suspected to decompose during quenching, it was important to perform this study in situ at the reaction temperature.
2. Experimental The catalyst used for the carbon growth consisted in cobalt particles supported by g-Al2O3. The preparation method of this catalyst has already been described in Ref. [15]. The cobalt content in the catalyst, as estimated by magnetic measurements, was approximately 6.4 wt% and the average size of the particles was about 5.7 nm. The nomenclature used for labeling the samples is as
follows: the prefix INS refers to samples prepared in situ while EXS corresponds to samples prepared ex situ. The reduced samples are designated by the suffix Red. A different suffix is used for the samples after reaction with CO depending on the percentage of H2 in the gas mixture: x H indicates that the gas mixture used for the carbon growth was containing x% of H2; 0H indicates that no hydrogen was present in the gas mixture. As the presence of CO2 in the gas mixture has no influence on the morphology of the carbon deposit, the same nomenclature will be used indifferently for pure CO or a CO/CO2 mixture. As an example, INS5H refers to a sample prepared in situ and reacted either with a CO/CO2/H2 or a CO/H2 mixture containing 5% of H2). The diffraction experiments were carried out at the European Synchrotron Radiation Facility powder diffraction beamline (ESRF/BM16, Grenoble, France)). Due to the small cobalt content in the catalyst, a high radiation flux was indeed required to emphasize the signal related to the metal phase. The high resolution and accuracy of the diffractometer at BM16 was also an advantage for crystallographic measurements since they allowed one to detect minor differences in the peak positions. In a general way, the data collection time is an important parameter when considering an in situ study. Beamline BM16 offered particularly appropriate conditions for an in situ study. The specific configuration of the diffractometer allowed one to greatly reduce the acquisition time for the diffraction patterns without compromising the angular accuracy. In routine operation, a bank of nine detectors was continuously scanned to measure the diffracted intensity as a function of 2u. Each detector only scanned a part of the whole angle range, which enabled to reduce by a factor 9 the acquisition time in comparison to a conventional diffractometer. In the present case, approximately 10 min was necessary for this configuration to perform a complete scan in the 1 – 338 2u-interval. ˚, For the in situ diffraction experiment at l ¼ 0:349117 A the catalyst powder was loaded into a borosilicate glass capillary tube with a diameter of 1.5 mm. This capillary tube was mounted on the 2-circle diffractometer. Both ends of the tube were connected with a heat shrinkable tube to the gas circuit. The capillary tube could, this way, be operated as a mini reactor wherein the gases were allowed to flow through. In order to avoid any powder displacement due to the gas pressure, the catalyst was maintained between two tufts of silica quartz wool. Heating of the capillary tube was performed using an air blower heater. The temperature in the tube was varied in the 500 – 600 8C range by adjusting the distance between the heater and the tube. Preliminary calibration tests were carried out to determine the appropriate distances. The compositions of the ingoing and outgoing gases were analyzed during the carbon growth by on-line gas chromatography. It was possible, this way, to follow the evolution of the reaction rate at the same time as the diffraction data were collected. Two separate runs were performed in situ. Both catalyst
J.P. Pinheiro et al. / Solid State Communications 123 (2002) 161–166
Fig. 2. HRTEM image of a carbon nanotube issued from the reaction of pure CO over Co/Al2O3 (sample INS0H ).
samples were reduced according to a similar procedure. The capillary tube containing the catalyst powder was first flushed with helium at room temperature for 15 min. Helium was then replaced by hydrogen and the capillary tube was progressively heated up to 600 8C (^10 8C). The catalysts were maintained under hydrogen flow at 600 8C for 2 h in order to reduce the cobalt oxides into cobalt. After reduction, the capillary tube was flushed with helium for at least 1 h at 600 8C in order to remove any residual hydrogen. According to the nomenclature previously described, INSRed1 and INSRed2 are the samples obtained after reduction in the first and the second run, respectively. After reduction, the catalyst powder INSRed1 was reacted with a CO/CO2 (96/4) mixture. The catalyst INSRed2 was treated with a CO/CO2 mixture containing a small amount of hydrogen (5%). INS0H and INS5H are the samples obtained in situ after reaction. In both cases, the carbon deposition reaction was pursued until complete deactivation of the catalyst (attested by the on-line chromatographic analyses). The catalyst deactivation was observed to occur more rapidly in the absence rather than in the presence of hydrogen. This influence of hydrogen is well known although its mechanism is still not well understood. Besides, the reaction rate was observed to be higher in presence of hydrogen. The conjunction of these two parameters (higher reaction rate and slower deactivation of the catalyst) resulted in the fact that a greater amount of
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carbon was deposited when hydrogen was added to the CO/CO2 mixture. Additional diffraction analyses were performed at BM16 ˚ on three samples prepared ex situ with at l ¼ 0:487760 A the same type of catalyst. Sample EXS0H was obtained by treating the cobalt catalyst in conditions similar to INS0H except that pure CO was used instead of a CO/CO2 mixture. The presence of CO2 in the gas mixture has been shown to have no influence on the morphology of the carbon deposit. It just affects the reaction kinetics. TEM observations confirmed the similarity of samples INS0H and EXS0H in terms of morphology of the carbon deposit. In both cases, only nanotubes were indeed observed. Samples EXS7H and EXS20H were produced by reacting the catalyst with CO/H2 mixtures containing different CO/H2 ratios (93:7 and 80:20, respectively). As for INS5H, the carbon deposit was shown to be exclusively composed of filaments. As previously said, the percentage of H2 has an influence on the inclination of the graphene layers. It was therefore important to measure d(002), i.e. how does this effect change the compactness of the atomic packing in the k001l direction. After reaction, all samples were observed by TEM. The microscope samples were prepared by sonicating the carbon deposits in ethyl alcohol and by dispersing them onto a holey carbon-coated copper electron microscope grid. The HRTEM observations were performed with a JEOL 200CX at 200 kV accelerating voltage.
3. Experimental results 3.1. HRTEM observations The observations confirmed that the presence of H2 influences the nanostructure of the carbon deposits. In accordance with our previous observations [5], the carbon deposits obtained in the absence of hydrogen (INS0H and EXS0H ) were exclusively constituted by nanotubes with diameters lower than 15 nm. The number of carbon layers, in this case, never exceeded 10 layers (Fig. 2). Conversely, each time that hydrogen was added to the CO/CO2 mixture (samples INS5H, EXS7H and EXS20H ), the carbon deposit was observed to be only composed of carbon filaments with diameters ranging from 10 to 30 nm (Fig. 3). 3.2. X-ray diffraction results
Fig. 3. HRTEM image of a carbon filament produced by the reaction of a CO/H2 mixture (95/5) over Co/Al2O3 (sample INS5H ).
For each run (with and without hydrogen), in situ diffraction analyses of the samples were performed consecutively during the reduction step and the carbon deposition reaction. The measured powder diffraction patterns are shown in Figs. 4 – 6. These patterns give information both on the evolution of the cobalt catalyst and
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Fig. 4. Diffraction pattern of the Co/Al2O3 catalyst after reduction (sample INSRed2 ). The cobalt peaks are designated by p . Open circles: experimental; solid line: calculated; bottom line: difference between observed and calculated diagrams.
Fig. 6. Diffraction pattern after reaction with a CO/H2 (95/5) mixture (sample INS5H ). Open circles: experimental; solid line: calculated; bottom line: difference between observed and calculated diagrams.
the structure of the carbon deposit. We shall discuss these two aspects separately in the following sections.
catalyst weight. The absence of peaks assigned to the cobalt oxides either CoO, Co3O4 or Co2O3 suggests that the reduction of the cobalt oxides was complete. In addition, the (111), (200), and (311) peaks of the fcc-cobalt can unambiguously be identified. This is the stable structure for cobalt over 422 8C. The lattice parameter of fcc-cobalt, as determined from the refinement of the diffraction line positions, is 0.3572 nm. In comparison, the value for bulk cobalt at room temperature is 0.3545 nm.
3.2.1. Evolution of the cobalt catalyst 3.2.1.1. After reduction. Fig. 4 shows a magnification of part of the diffraction profile of INSRed2, and compares the experimental and the calculated diffraction patterns. The diffraction patterns obtained after reduction for each run (INSRed1 and INSRed2 ) are very similar. The calculated pattern was obtained using the whole pattern refinement program FULLPROF [16], assuming the presence of g-Al2O3 and fcc-Co. The diffraction pattern is dominated by the diffraction peaks imputable to alumina, which is not surprising since cobalt accounts only for 6.4% of the total
Fig. 5. Diffraction pattern after reaction with pure CO (sample INS0H ). Open circles: experimental; solid line: calculated; bottom line: difference between observed and calculated diagrams.
3.2.1.2. After reaction. The main change in comparison with the diffraction pattern of the reduced catalyst was, for both runs (INS0H and INS5H ), the apparition of new peaks, attributable to carbon. A satisfying fit of the experimental patterns was obtained by taking into account three phases: g-Al2O3, fcc-cobalt, and carbon. As the carbon nanotubes and filaments are expected to have a turbostratic structure, the refinement of the carbon scattering was performed by using the ‘in-plane’ parameters derived from those of graphite ða ¼ 0:246 nmÞ and a variable d(002) lattice spacing. Figs. 5 and 6 show the good agreement between the calculated and the experimental diffraction patterns. It is worth noting that although the experiments were conducted until complete deactivation of the catalyst, no evolution of the cobalt phase was observed. As previously said, deactivation is generally attributed to two distinct phenomena (carbide formation and/or carbon coating). In the present case, the absence of any new cobalt phase could therefore suggest that the catalyst deactivation is related to the carbon coating of the particles. This interpretation must, however, be considered with caution. A carburization of the particle surface would be, indeed, sufficient to induce a complete deactivation of the particle, although a surface carbide phase could still remain undetectable by X-ray diffraction. In view of the complexity of the diffraction
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patterns, it cannot indeed be excluded that the diffraction signals corresponding to minor phases (including eventually a carbide phase) are masked by the more intense peaks due to the predominant phases. The only certain conclusion therefore is that the deactivation of the catalyst particle is not the result of bulk carburization. It is worth noting that the cobalt peaks have the same position in the diffraction patterns of the ex situ samples (measured at room temperature) as in the patterns of in situ. This result demonstrates that the cobalt particles still have an fcc structure at room temperature. The stability of this structure at room temperature has already been reported earlier [17,18]. Ram [19], in particular, observed that cobalt nanoparticles adopt a fcc or a bcc structure rather than the hcp bulk structure in order to minimize their surface and internal energy. 3.2.2. Structure of the carbon deposit. The determination of the lattice parameters of the carbon deposit was complicated by the fact that most carbon peaks were overlapped with peaks originating from other phases. As visible in INS0H and INS5H diffraction patterns (Figs. 5 and 6), the (002) peak is superposed with the signal imputable to the capillary tube. This overlap is particularly perceptible for the experiment performed without hydrogen for which a lesser amount of carbon was deposited (i.e. INS0H ). In view of the observed small variations in d(002), it is important to evaluate the uncertainty of the peak position measurements. The main sources for inexactness of d(002) lie in the zero error of the 2u scale and in the process for discriminating and fitting the adjacent capillary and carbon peaks and the background. (1) Regarding the 2u scale, the peak fitting was also performed on the carbon (10) line whose position was not expected to vary. The profile fitting of the EXS0H, EXS7H, and EXS20H patterns shows that three peaks are present in the region 12.7– 15.38, corresponding, respectively, to the (10) peak of carbon, to the (111) line of fcc-cobalt and to the (400) reflection of the g-Al2O3 matrix. The diffraction lines were fitted with a symmetric pseudo-Voigt profile function. For the (10) line, this approximation is, however, not completely appropriate. Due to the turbostratic nature of the carbon nanotubes and filaments, this line is indeed expected to have an asymmetric shape. It can be assumed, however, that this approximation has only a negligible impact on the position of the (10) line or, more precisely, that it has a similar influence for the three diffraction patterns. The position of the (10) line shifts by less than 0.0058 2u for the three ex situ samples, i.e. Dd/d is of the order of 0.04%. (2) Regarding the reliability of the fitting, we compared the values given by two independent fitting methods. The first procedure disentangles the partially overlapping peaks of the capillary tube and of the carbon phase using a symmetric pseudo-Voigt profile function. The parameters for the capillary signal (position, intensity, FWHM) were determined directly from the reduced catalyst diffraction
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patterns (INSRed1 and INSRed2 ). The second procedure assumes the peaks are rather symmetric and hence compares the 2u shift between the peak centroid and the peak maximum versus the relative amplitudes and overlapping of the neighboring capillary and (002) carbon peaks. The mismatch between both methods is always less than 0.6%. The determination of the (002) line position by both methods gives significantly different results for the carbon nanotubes and the carbon filaments for both in situ as well as ex situ samples. The measured relative variations of the (002) peak position for these samples are significantly larger than the experimental and fitting error bar. The error bar is quoted between brackets for the values given later. The position of the (002) line for INS0H (nanotubes) and INS5H (filaments), respectively, corresponds to an interlayer spacing of 0.349(1) and 0.347(1) nm at 600 8C. This seems to demonstrate that the (002) lattice spacing is greater for the carbon nanotubes than for the filaments. The (002) lattice parameter at room temperature is, respectively, equal to 0.343(1), 0.339(2), and 0.338(2) nm for the ex situ samples EXS0H, EXS7H and EXS20H. Therefore, the evolution of the (002) inter-layer spacing for the three samples EXS0H, EXS7H and EXS20H is similar to that previously observed for the in situ samples: the distance between the graphitic sheets is larger for the nanotubes than for the filaments by approximately 1.5% at the most. In view of the available information, it is, however, still premature to conclude whether the d(002) variations are related to structural or size differences between the carbon nanotubes and filaments. The TEM observations have indeed shown that the carbon filaments have larger diameters than the carbon nanotubes. According to Kiang et al. [10], the d(002) inter-layer spacing is expected to decrease with increasing tube diameter. In the present case, it cannot therefore be excluded that the differences in d(002) are due to a similar size effect.
4. Conclusions The structural differences between the carbon nanotubes and filaments were investigated by X-ray diffraction using synchrotron X-ray radiation. In situ measurements performed during carbon growth at 600 8C demonstrate that the (002) inter-layer spacing in carbon nanotubes (0.3486 nm) is substantially larger than that of carbon filaments (0.3469 nm). A similar trend of d(002) is observed when the stacking geometry of the carbon layers changes from nanotube to filament in ex situ samples at room temperature. The implication of cobalt carburization in the catalyst deactivation during carbon growth was also investigated. The results of this study demonstrate that the deactivation of the catalyst particles is not the result of bulk carburization.
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References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] W.A. de Heer, A. Chaˆtelain, D. Ugarte, Science 270 (1995) 1179. [3] T.W. Odom, J.L. Huang, P. Kim, C.M. Lieber, Nature 391 (1997) 62–64. [4] R.T.K. Baker, N. Rodriguez, Mater. Res. Soc. Symp. Proc. 349 (1994) 251–256. [5] J.P. Pinheiro, M.C. Schouler, P. Gadelle, M. Mermoux, E. Dooryhee, Carbon 38 (2000) 1469–1479. [6] S. Iijima, Nature 354 (1991) 56–58. [7] Y. Saito, T. Yoshikawa, S. Bandow, M. Tomita, T. Hayashi, Phys. Rev. B 48 (1993) 1907– 1909. [8] M. Bretz, B.G. Demczyk, L. Zhang, J. Cryst. Growth 141 (1994) 304. [9] X. Sun, C.H. Kiang, M. Endo, K. Takeuchi, T. Furuta, M.S. Dresselhaus, Phys. Rev. B 54 (1996) 12629–12632. [10] C.H. Kiang, M. Endo, P.M. Ajayan, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. Lett. 81 (1998) 1869–1872.
[11] R.T.K. Baker, J.R. Alonzo, J.A. Dumesic, D.J.C. Yates, J. Catal. 77 (1982) 74–84. [12] S. Herreyre, P. Gadelle, P. Moral, J.M.M. Millet, J. Phys. Chem. Solids 58 (1997) 1539–1545. [13] J.P. Pinheiro, P. Gadelle, J. Phys. Chem. Solids 62 (2001) 1015–1021. [14] J.P. Pinheiro, P. Gadelle, C. Jeandey, J.L. Oddou, J. Phys. Chem. Solids 62 (2001) 1023–1037. [15] A. Thaib, G.A. Martin, P. Pinheiro, M.C. Schouler, P. Gadelle, Catal. Lett. 63 (1999) 135–141. [16] J. Rodriguez-Carvajal, FULLPROF , Satellite Meeting on Powder Diffraction at the XVth Congress of the I.U.Cr, Toulouse, France, 1990, pp. 127–128. [17] S. Liu, X. Tang, L. Yin, Y. Koltypin, A. Gedanken, J. Mater. Chem. 10 (2000) 1271. [18] R.K. Rana, Y. Koltypin, A. Gedanken, Chem. Phys. Lett. 344 (2001) 256 –262. [19] S. Ram, Mater. Sci. Engng A 304 –306 (2001) 923 –927.
In situ X-ray diffraction study of carbon nanotubes and filaments during their formation over Co/Al2O3 catalysts J.P. Pinheiroa,*, M.C. Schoulera, E. Dooryheeb a
Laboratoire de Thermodynamique et Physico-Chimie Me´tallurgiques, UMR 5614 CNRS-INPG-UJF, Grenoble, France Laboratoire de Cristallographie, CNRS UPR5031, 25 avenue des Martyrs, BP 166, 38042 Grenoble Cedex 9, France
b
Received 25 March 2002; accepted 28 April 2002 by P. Burlet
Abstract Depending on the experimental conditions, the CO disproportionation reaction over well-calibrated Co/Al2O3 catalysts leads to the formation of carbon with two distinct morphologies: carbon nanotubes (with cylindrical carbon layers parallel to the tube axis) and carbon filaments (constituted of carbon layers with a truncated cone shape). The structural differences between these two morphologies were investigated by X-ray diffraction using synchrotron X-ray radiation. In situ X-ray diffraction analyses performed during the carbon growth at 600 8C show that the (002) inter-layer spacing is larger by a few percents in carbon nanotubes than in carbon filaments. d(002) is, respectively, equal to 0.3486 and 0.3469 nm for the nanotubes and filaments. The variation of d(002) with the graphene layer stacking geometry was confirmed by additional measurements performed at room temperature on samples prepared ex situ. The evolution of the cobalt catalyst during the carbon growth was also investigated. The results of this study indicate that the catalyst deactivation is not the result of a bulk transformation of the cobalt particles. q 2002 Elsevier Science Ltd. All rights reserved. PACS: 81.07.De; 61.12.Ld Keywords: A. Carbon nanotab; A. Carbon filament; B. Catalytic growth; C. X-ray scattering; E. Synchrotron radiation
1. Introduction Due to their remarkable properties, carbon nanotubes show promise for numerous potential applications including gas absorption, field emission in flat-panel displays or use as nanoscale electronic devices [1– 3]. Among all the methods currently used for nanotube preparation, the chemical vapor deposition techniques (CVD, HFCVD, etc.) are probably the most promising for large-scale, low-cost production of such structures. A major feature of these techniques is that they allow the controlled production of a wide variety of shapes and morphologies [4]. In the present work, we will mainly focus on two kinds of carbon morphologies: nanotubes and filaments. We shall call nanotubes carbon structures in which the carbon layers consist in coaxial cylinders and carbon filaments structures in which the carbon layers have a truncated cone shape (Fig. 1). * Corresponding author. E-mail address: [email protected] (J.P. Pinheiro).
In a previous study [5], we demonstrated that it is possible to select the nature of the carbon structure by applying appropriate gas mixtures on well-calibrated Co/ Al2O3 catalyst between 500 and 600 8C. This work showed indeed that the carbon deposit issued from disproportionation of pure CO was exclusively composed of nanotubes. Conversely, when H2 was added to CO, only filaments were produced. It was observed, in that case, that the inclination between the carbon layers and the tube axis depends on the CO/H2 ratio and can be increased by increasing the amount of hydrogen in the mixture. It is to be noted that mixtures of both morphologies were never observed. Since their discovery by Iijima in the early 1990s [6], the carbon nanotubes have been the subject of many structural studies. X-ray diffraction studies performed by Saito et al. [7] revealed that the distance between the carbon layers in these structures was larger by a few percents than that in bulk graphite. Saito et al. estimated that the average intershell spacing is equal to 0.344 nm. High resolution transmission electron microscopy (HRTEM) and electron
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 0 9 8 ( 0 2 ) 0 0 1 8 9 - 8
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Fig. 1. Structure of the carbon nanotubes and carbon filaments.
diffraction studies conducted by Bretz et al. [8] and Sun et al. [9] gave a larger estimation of this distance (0.375 and 0.360 nm, respectively). This disparity in values was interpreted as the result of a possible dependence on the tube size by Kiang et al. [10], who proposed a model taking into account the nanotube diameter. It is to be noted that the existing diffraction data essentially deal with carbon nanotubes and that only a limited amount of information is available on filaments. The first objective of the present study is therefore to measure how the d(002) lattice spacing varies with the graphene layer stacking geometry. A progressive deactivation of the catalyst is generally observed during the carbon growth when CO is used as the carbon-bearing gas. In the case of iron-containing catalysts, it has been demonstrated that carburization (formation of Fe3C) is strongly involved in the deactivation process [11, 12]. Some authors also attribute the deactivation to an additional phenomenon: the carbon coating over the surface of the catalytic particle [13,14]. This surface carbon layer would hinder the contact between the gas phase and the particle and would therefore have a negative influence on the catalyst activity. The second objective of the present study is to search for the possible transformation of the cobalt particles into any cobalt carbide phase in the case of a cobalt-containing catalyst. Since the cobalt carbides are suspected to decompose during quenching, it was important to perform this study in situ at the reaction temperature.
2. Experimental The catalyst used for the carbon growth consisted in cobalt particles supported by g-Al2O3. The preparation method of this catalyst has already been described in Ref. [15]. The cobalt content in the catalyst, as estimated by magnetic measurements, was approximately 6.4 wt% and the average size of the particles was about 5.7 nm. The nomenclature used for labeling the samples is as
follows: the prefix INS refers to samples prepared in situ while EXS corresponds to samples prepared ex situ. The reduced samples are designated by the suffix Red. A different suffix is used for the samples after reaction with CO depending on the percentage of H2 in the gas mixture: x H indicates that the gas mixture used for the carbon growth was containing x% of H2; 0H indicates that no hydrogen was present in the gas mixture. As the presence of CO2 in the gas mixture has no influence on the morphology of the carbon deposit, the same nomenclature will be used indifferently for pure CO or a CO/CO2 mixture. As an example, INS5H refers to a sample prepared in situ and reacted either with a CO/CO2/H2 or a CO/H2 mixture containing 5% of H2). The diffraction experiments were carried out at the European Synchrotron Radiation Facility powder diffraction beamline (ESRF/BM16, Grenoble, France)). Due to the small cobalt content in the catalyst, a high radiation flux was indeed required to emphasize the signal related to the metal phase. The high resolution and accuracy of the diffractometer at BM16 was also an advantage for crystallographic measurements since they allowed one to detect minor differences in the peak positions. In a general way, the data collection time is an important parameter when considering an in situ study. Beamline BM16 offered particularly appropriate conditions for an in situ study. The specific configuration of the diffractometer allowed one to greatly reduce the acquisition time for the diffraction patterns without compromising the angular accuracy. In routine operation, a bank of nine detectors was continuously scanned to measure the diffracted intensity as a function of 2u. Each detector only scanned a part of the whole angle range, which enabled to reduce by a factor 9 the acquisition time in comparison to a conventional diffractometer. In the present case, approximately 10 min was necessary for this configuration to perform a complete scan in the 1 – 338 2u-interval. ˚, For the in situ diffraction experiment at l ¼ 0:349117 A the catalyst powder was loaded into a borosilicate glass capillary tube with a diameter of 1.5 mm. This capillary tube was mounted on the 2-circle diffractometer. Both ends of the tube were connected with a heat shrinkable tube to the gas circuit. The capillary tube could, this way, be operated as a mini reactor wherein the gases were allowed to flow through. In order to avoid any powder displacement due to the gas pressure, the catalyst was maintained between two tufts of silica quartz wool. Heating of the capillary tube was performed using an air blower heater. The temperature in the tube was varied in the 500 – 600 8C range by adjusting the distance between the heater and the tube. Preliminary calibration tests were carried out to determine the appropriate distances. The compositions of the ingoing and outgoing gases were analyzed during the carbon growth by on-line gas chromatography. It was possible, this way, to follow the evolution of the reaction rate at the same time as the diffraction data were collected. Two separate runs were performed in situ. Both catalyst
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Fig. 2. HRTEM image of a carbon nanotube issued from the reaction of pure CO over Co/Al2O3 (sample INS0H ).
samples were reduced according to a similar procedure. The capillary tube containing the catalyst powder was first flushed with helium at room temperature for 15 min. Helium was then replaced by hydrogen and the capillary tube was progressively heated up to 600 8C (^10 8C). The catalysts were maintained under hydrogen flow at 600 8C for 2 h in order to reduce the cobalt oxides into cobalt. After reduction, the capillary tube was flushed with helium for at least 1 h at 600 8C in order to remove any residual hydrogen. According to the nomenclature previously described, INSRed1 and INSRed2 are the samples obtained after reduction in the first and the second run, respectively. After reduction, the catalyst powder INSRed1 was reacted with a CO/CO2 (96/4) mixture. The catalyst INSRed2 was treated with a CO/CO2 mixture containing a small amount of hydrogen (5%). INS0H and INS5H are the samples obtained in situ after reaction. In both cases, the carbon deposition reaction was pursued until complete deactivation of the catalyst (attested by the on-line chromatographic analyses). The catalyst deactivation was observed to occur more rapidly in the absence rather than in the presence of hydrogen. This influence of hydrogen is well known although its mechanism is still not well understood. Besides, the reaction rate was observed to be higher in presence of hydrogen. The conjunction of these two parameters (higher reaction rate and slower deactivation of the catalyst) resulted in the fact that a greater amount of
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carbon was deposited when hydrogen was added to the CO/CO2 mixture. Additional diffraction analyses were performed at BM16 ˚ on three samples prepared ex situ with at l ¼ 0:487760 A the same type of catalyst. Sample EXS0H was obtained by treating the cobalt catalyst in conditions similar to INS0H except that pure CO was used instead of a CO/CO2 mixture. The presence of CO2 in the gas mixture has been shown to have no influence on the morphology of the carbon deposit. It just affects the reaction kinetics. TEM observations confirmed the similarity of samples INS0H and EXS0H in terms of morphology of the carbon deposit. In both cases, only nanotubes were indeed observed. Samples EXS7H and EXS20H were produced by reacting the catalyst with CO/H2 mixtures containing different CO/H2 ratios (93:7 and 80:20, respectively). As for INS5H, the carbon deposit was shown to be exclusively composed of filaments. As previously said, the percentage of H2 has an influence on the inclination of the graphene layers. It was therefore important to measure d(002), i.e. how does this effect change the compactness of the atomic packing in the k001l direction. After reaction, all samples were observed by TEM. The microscope samples were prepared by sonicating the carbon deposits in ethyl alcohol and by dispersing them onto a holey carbon-coated copper electron microscope grid. The HRTEM observations were performed with a JEOL 200CX at 200 kV accelerating voltage.
3. Experimental results 3.1. HRTEM observations The observations confirmed that the presence of H2 influences the nanostructure of the carbon deposits. In accordance with our previous observations [5], the carbon deposits obtained in the absence of hydrogen (INS0H and EXS0H ) were exclusively constituted by nanotubes with diameters lower than 15 nm. The number of carbon layers, in this case, never exceeded 10 layers (Fig. 2). Conversely, each time that hydrogen was added to the CO/CO2 mixture (samples INS5H, EXS7H and EXS20H ), the carbon deposit was observed to be only composed of carbon filaments with diameters ranging from 10 to 30 nm (Fig. 3). 3.2. X-ray diffraction results
Fig. 3. HRTEM image of a carbon filament produced by the reaction of a CO/H2 mixture (95/5) over Co/Al2O3 (sample INS5H ).
For each run (with and without hydrogen), in situ diffraction analyses of the samples were performed consecutively during the reduction step and the carbon deposition reaction. The measured powder diffraction patterns are shown in Figs. 4 – 6. These patterns give information both on the evolution of the cobalt catalyst and
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Fig. 4. Diffraction pattern of the Co/Al2O3 catalyst after reduction (sample INSRed2 ). The cobalt peaks are designated by p . Open circles: experimental; solid line: calculated; bottom line: difference between observed and calculated diagrams.
Fig. 6. Diffraction pattern after reaction with a CO/H2 (95/5) mixture (sample INS5H ). Open circles: experimental; solid line: calculated; bottom line: difference between observed and calculated diagrams.
the structure of the carbon deposit. We shall discuss these two aspects separately in the following sections.
catalyst weight. The absence of peaks assigned to the cobalt oxides either CoO, Co3O4 or Co2O3 suggests that the reduction of the cobalt oxides was complete. In addition, the (111), (200), and (311) peaks of the fcc-cobalt can unambiguously be identified. This is the stable structure for cobalt over 422 8C. The lattice parameter of fcc-cobalt, as determined from the refinement of the diffraction line positions, is 0.3572 nm. In comparison, the value for bulk cobalt at room temperature is 0.3545 nm.
3.2.1. Evolution of the cobalt catalyst 3.2.1.1. After reduction. Fig. 4 shows a magnification of part of the diffraction profile of INSRed2, and compares the experimental and the calculated diffraction patterns. The diffraction patterns obtained after reduction for each run (INSRed1 and INSRed2 ) are very similar. The calculated pattern was obtained using the whole pattern refinement program FULLPROF [16], assuming the presence of g-Al2O3 and fcc-Co. The diffraction pattern is dominated by the diffraction peaks imputable to alumina, which is not surprising since cobalt accounts only for 6.4% of the total
Fig. 5. Diffraction pattern after reaction with pure CO (sample INS0H ). Open circles: experimental; solid line: calculated; bottom line: difference between observed and calculated diagrams.
3.2.1.2. After reaction. The main change in comparison with the diffraction pattern of the reduced catalyst was, for both runs (INS0H and INS5H ), the apparition of new peaks, attributable to carbon. A satisfying fit of the experimental patterns was obtained by taking into account three phases: g-Al2O3, fcc-cobalt, and carbon. As the carbon nanotubes and filaments are expected to have a turbostratic structure, the refinement of the carbon scattering was performed by using the ‘in-plane’ parameters derived from those of graphite ða ¼ 0:246 nmÞ and a variable d(002) lattice spacing. Figs. 5 and 6 show the good agreement between the calculated and the experimental diffraction patterns. It is worth noting that although the experiments were conducted until complete deactivation of the catalyst, no evolution of the cobalt phase was observed. As previously said, deactivation is generally attributed to two distinct phenomena (carbide formation and/or carbon coating). In the present case, the absence of any new cobalt phase could therefore suggest that the catalyst deactivation is related to the carbon coating of the particles. This interpretation must, however, be considered with caution. A carburization of the particle surface would be, indeed, sufficient to induce a complete deactivation of the particle, although a surface carbide phase could still remain undetectable by X-ray diffraction. In view of the complexity of the diffraction
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patterns, it cannot indeed be excluded that the diffraction signals corresponding to minor phases (including eventually a carbide phase) are masked by the more intense peaks due to the predominant phases. The only certain conclusion therefore is that the deactivation of the catalyst particle is not the result of bulk carburization. It is worth noting that the cobalt peaks have the same position in the diffraction patterns of the ex situ samples (measured at room temperature) as in the patterns of in situ. This result demonstrates that the cobalt particles still have an fcc structure at room temperature. The stability of this structure at room temperature has already been reported earlier [17,18]. Ram [19], in particular, observed that cobalt nanoparticles adopt a fcc or a bcc structure rather than the hcp bulk structure in order to minimize their surface and internal energy. 3.2.2. Structure of the carbon deposit. The determination of the lattice parameters of the carbon deposit was complicated by the fact that most carbon peaks were overlapped with peaks originating from other phases. As visible in INS0H and INS5H diffraction patterns (Figs. 5 and 6), the (002) peak is superposed with the signal imputable to the capillary tube. This overlap is particularly perceptible for the experiment performed without hydrogen for which a lesser amount of carbon was deposited (i.e. INS0H ). In view of the observed small variations in d(002), it is important to evaluate the uncertainty of the peak position measurements. The main sources for inexactness of d(002) lie in the zero error of the 2u scale and in the process for discriminating and fitting the adjacent capillary and carbon peaks and the background. (1) Regarding the 2u scale, the peak fitting was also performed on the carbon (10) line whose position was not expected to vary. The profile fitting of the EXS0H, EXS7H, and EXS20H patterns shows that three peaks are present in the region 12.7– 15.38, corresponding, respectively, to the (10) peak of carbon, to the (111) line of fcc-cobalt and to the (400) reflection of the g-Al2O3 matrix. The diffraction lines were fitted with a symmetric pseudo-Voigt profile function. For the (10) line, this approximation is, however, not completely appropriate. Due to the turbostratic nature of the carbon nanotubes and filaments, this line is indeed expected to have an asymmetric shape. It can be assumed, however, that this approximation has only a negligible impact on the position of the (10) line or, more precisely, that it has a similar influence for the three diffraction patterns. The position of the (10) line shifts by less than 0.0058 2u for the three ex situ samples, i.e. Dd/d is of the order of 0.04%. (2) Regarding the reliability of the fitting, we compared the values given by two independent fitting methods. The first procedure disentangles the partially overlapping peaks of the capillary tube and of the carbon phase using a symmetric pseudo-Voigt profile function. The parameters for the capillary signal (position, intensity, FWHM) were determined directly from the reduced catalyst diffraction
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patterns (INSRed1 and INSRed2 ). The second procedure assumes the peaks are rather symmetric and hence compares the 2u shift between the peak centroid and the peak maximum versus the relative amplitudes and overlapping of the neighboring capillary and (002) carbon peaks. The mismatch between both methods is always less than 0.6%. The determination of the (002) line position by both methods gives significantly different results for the carbon nanotubes and the carbon filaments for both in situ as well as ex situ samples. The measured relative variations of the (002) peak position for these samples are significantly larger than the experimental and fitting error bar. The error bar is quoted between brackets for the values given later. The position of the (002) line for INS0H (nanotubes) and INS5H (filaments), respectively, corresponds to an interlayer spacing of 0.349(1) and 0.347(1) nm at 600 8C. This seems to demonstrate that the (002) lattice spacing is greater for the carbon nanotubes than for the filaments. The (002) lattice parameter at room temperature is, respectively, equal to 0.343(1), 0.339(2), and 0.338(2) nm for the ex situ samples EXS0H, EXS7H and EXS20H. Therefore, the evolution of the (002) inter-layer spacing for the three samples EXS0H, EXS7H and EXS20H is similar to that previously observed for the in situ samples: the distance between the graphitic sheets is larger for the nanotubes than for the filaments by approximately 1.5% at the most. In view of the available information, it is, however, still premature to conclude whether the d(002) variations are related to structural or size differences between the carbon nanotubes and filaments. The TEM observations have indeed shown that the carbon filaments have larger diameters than the carbon nanotubes. According to Kiang et al. [10], the d(002) inter-layer spacing is expected to decrease with increasing tube diameter. In the present case, it cannot therefore be excluded that the differences in d(002) are due to a similar size effect.
4. Conclusions The structural differences between the carbon nanotubes and filaments were investigated by X-ray diffraction using synchrotron X-ray radiation. In situ measurements performed during carbon growth at 600 8C demonstrate that the (002) inter-layer spacing in carbon nanotubes (0.3486 nm) is substantially larger than that of carbon filaments (0.3469 nm). A similar trend of d(002) is observed when the stacking geometry of the carbon layers changes from nanotube to filament in ex situ samples at room temperature. The implication of cobalt carburization in the catalyst deactivation during carbon growth was also investigated. The results of this study demonstrate that the deactivation of the catalyst particles is not the result of bulk carburization.
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